Space-based laser communication systems establish and maintain constant spatial orientation between satellites for providing a continuous closed-loop communications link. A ground system will search for a space system initiating a laser communication link with the ground system. The space system directs a laser beam at an electro-optical focal plane array sensor of the ground system. A focal plane array is known as an imaging sensor. A detected pixel is a pixel having a magnitude value of pixel data exceeding a detection threshold level. The pixel data is a decimal value of the magnitude of the impinging light on a pixel in the array. The pixel data is a decimal value equal to or exceeding the predetermined detection threshold level. Pixel data is typically any number of N parallel bits. The pixel data is typically used to establish and maintain a communications link. The ground system must acquire, detect and then track by signal processing the laser beam of the space system in a continuous and closed-loop process. The ground system acquires continuous frames of image data as long as the laser is impinging the sensor. Each frame may consist of millions of pixels. Each pixel is associated with a respective row and column in the focal plane array. These systems may be space satellites. Two satellites may attempt to initiate, establish, and maintain an efficient and optimal laser communication link. Each satellite must be able to independently sense and acquire the presence of the laser light. That is, each satellite must be able to detect the laser light to initiate a communications link, and subsequently maintain continuous tracking of that laser light originating from the satellite attempting to initiate the communication link to ensure unbroken connectivity of that link. The sensor may be a complementary metal oxide semiconductor (CMOS) focal plane array, which would be continuously scanning a portion of space within a field of view searching for the laser beam where the satellite may be positioned. Once a laser signal, originating from the space satellite, impinges on the photodetector array surface of the imaging sensor of the ground system, output data from that sensor can then be processed by a signal processing system. The output data from the signal processing system would then be sent to a positing controller for commanding a pointing mechanism to move the sensor field of view as the sensor is continuously moved so as to continuously track the laser beam. Continuing technological advancements in satellite imaging sensors are now making these focal plane array sensors much larger with multi-mega pixels. The sensors have more complex structures and operate at even faster data rates, which places further stringent performance requirements on signal processing speeds to attain faster pointing response times, more precise pointing command resolution, and ever faster closed loop system response times.
The beamwidth of the impinging laser beam is affected and influenced by space phenomenology. The laser beam propagates from the originating satellite where the laser beam originates, to the receiving satellite on which the focal plane array imaging sensor resides. The spot beam is affected by physical interaction of the impinging laser beam with the surface of the imaging sensor, and any related and ensuing motion between the impinging laser beam and sensor surface. This motion is laser jitter and disadvantageously affects the ability of a tracking system to smoothly track the laser beam source.
The positioning of the sensor operates by continuous closed-loop control. That is, the signal processing system continuously accepts the imaging sensor outputs to calculate and update the impinging laser light on respective X and Y Cartesian coordinate centroid locations with respect to the illuminated spot of laser light illuminating individual pixels of the focal plane array. The signal processing system then outputs pointing commands to the positioning controller for controlling the pointing mechanism. In this way, continuous tracking of the impinging laser is maintained. When no laser beam is impinging on the focal plane array, the signal processing system is still accepting and processing inputs from the focal plane array while the focal plane array is continuously scanning over the field of view while awaiting for the arrival of a laser signal. When no laser beam is received, the pointing mechanism receives no updated pointing commands from the signal processor system so the focal plane array remains fixed in the position or the signal processing system can enter a scanning mode searching for a signal.
Various sensor control systems have been used. U.S. Pat. No. 6,469,815 teaches utilizing a quadrant sensor that is a multi-channel photo-detector. Each of the four sensor quadrants requires a dedicated processing channel. The received laser signal also requires conversion into electrical signals and threshold circuits, which supply thresholds for the converted electrical signal to be compared against. This system is for acquisition of the laser signal only. The spacecraft requires a second sensor to perform laser tracking. U.S. Pat. No. 6,522,440 also teaches the use of a four-quadrant photo-detector and respective channel processing circuitry and fine laser tracking is accomplished by relative power comparisons. U.S. Pat. No. 5,142,400 teaches two different sensors in a transceiver using a matched pair of reflecting telescopes and processing electronics and software for the comparison of two different optical beams for detection, acquisition, and tracking of the laser. U.S. Pat. No. 5,973,310 teaches three embodiments necessitating the use of binning of signals received from the imaging sensor. In the case the imaging sensor, the sensor is a charged coupled device. The sensor requires the use of a modified discrete Fourier transform of the binned signals and phase relationships. The sensor requires a finite impulse response transform with phase relationships, requires the collection of N image samples of the binned signals and calculation of an error angle, and lastly requires an iterative and repetitive methodology used for reducing window size until the signal is located. U.S. Pat. No. 6,556,324 teaches the use of separate sensors for the respective acquisition and tracking functions as well as three assemblies to implement the functions of point ahead, coarse, and fine pointing. U.S. Pat. No. 4,979,221 teaches a system where pixel data is used to create respective binary addresses for locating a given pixel on a charge coupled device array and sub-pixel centroiding. U.S. Pat. No. 5,517,016 teaches the storage and comparisons between three stored sets of collected pixel brightness magnitude data for further processing as well as requiring the determination of a relative angle between receive and transmit laser beams. U.S. Pat. No. 5,517,016 teaches the use of multiple radial compare window frames of data and uses a number of pixels to make logic decisions. U.S. Pat. No. 5,592,320 teaches scanning logic, scan controller, and signal processor elements as segregated system elements.
These cited patents teach that separate sensors are required for implementing the respective laser acquisition and tracking functions. The cited patents teach individual sensors necessitate additional processing hardware. The cited patents teach software and logic resources required for the acquisition sensor to cue the tracking sensor while incurring increased system size, weight, power, thermal control, and complexity. The cited patents teach the use of additional hardware and software for storage and preprocessing of sensor data for centroid processing and calculations, teach expensive and complex optical signal processing and components, optical beam power splitting, and optical beam power measurements. The optical components are sensitive to space radiation and thermal environments, suffer aging effects, maintaining mechanical alignment and calibration is critical. The cited patents also teach custom and fixed nonmodifiable system designs for any given satellite. The cited patents teach hardware intensive based implementations rather than reprogrammable software. The cited patents teach hardware intensive implementations require more control and monitoring by the signal processing system that has overall responsibility for the control and monitoring the functions of all other individual subsystems that make up the total satellite. The cited patents also teach centroid calculations used for data comparing and repetitive, iterative processes to eventually converge on X and Y values, rather than operating in real-time, or on-the-fly mode of operation.
There are critical problems with the teachings of the cited references. The operational limitations and shortfalls are due to slow signal processing and the obtainable precision of the pointing commands that the signal processing system uses to generate for the pointing commands. This signal processing time directly impacts the sensor positioning and repositioning response time that is defined as the time required for the sensor pointing mechanism and positioning controller to accept and respond to new pointing commands and to then point to that commanded location. The sensor pointing response time is extremely critical, as the response time is a dominant factor to ensure a continuous and highly accurate tracking of the impinging laser beam. Typical values of signal processing time, as measured between the signal processing mechanism accepting inputs from the imaging sensor, calculating the X and Y Cartesian coordinate centroid values to the final output of pointing commands to the sensor's pointing mechanism and positioning controller is on the order of milliseconds. Slow signal processing time and inadequately low pointing resolution of the generated pointing commands are problems of the closed-loop system response. In considering the imaging sensor, the signal processing mechanism and the sensor pointing mechanism and positioning controller as a single, closed-loop system, the overall system response of the closed-loop system is approximately on the order of 30.0 Hz. This overall response time is too slow and as such, will not accommodate the requirements of advanced imaging sensors.
Another problem is that the prior systems do not implement an efficient and optimal use of the imaging sensor built-in and primary operating feature. This feature is called the frame rate. Satellite imaging sensors with focal plane arrays, operate in one of two modes. When the focal plane array is operating in the full frame mode, all the individual pixels of that focal plane array surface are examined for any impinging laser light. This scanning process is done internally by the focal plane array internal electronics. The focal plane array will output pixel data and specialized control signals whether a laser is impinging the surface or not. When no laser light is present, then the pixel data is then zero. The specialized control signals will be different for each of the focal plane arrays made by respective manufacturers. These specialized control signals are used for timing control and sequencing for signal processing of the focal plane array pixel data. The full frame mode is typically used for laser signal detection and acquisition, when the focal plane array is initially staring into a field of view while waiting for a laser signal to impinge on the array. The second mode of focal plane array operation is a window of interest mode. The window of interest mode is predefined a priori within the signal processor to output a smaller N×N pixel subset of the focal plane array pixels. The cited references teach, in terms of speed, accuracy and precision, a signal processor that reduces the full scan to a fixed-sized window of interest of a fixed number of pixels. As such, the scanning limits system response time.
Another problem with prior system is the use of quadrature photodetector cells as an imaging sensor, be it for either for laser detection, acquisition or tracking functions. These quadrature photodetector cells devices offer a very limited small field-of-views in terms of how much space the sensor can scan through at any given time and the outputs from these quadrature photodetector cells devices are error signals. Error signals only provide the signal processor with limited data in which even further processing is required, which then allows a less precise coarse laser tracking function. Quadrant detectors or quad cells have been the principal means of achieving precise sensing of laser beams within an acquisition and tracking scheme. A drawback to a quad cell implementation is a highly restricted field of view for the four cell quadrant detectors. Precise optical alignment between the sensor and laser beam line-of-sight is not always achievable. The use of wide-angle field-of-view detectors allows for simplification of apparatus design and a decreased latency associated with switching between acquisition and tracking.
The use of a two-dimensional CMOS focal plane array as a detector device, exhibits superior performance for certain region-of-interest as compared to the equivalent charge coupled device detector arrays. Current advances in sub-micron feature CMOS technology have been exploited by developers of visible detector arrays to provide features, such as micron sized pixel dimensions, random access readout and image windowing, which are not available using alternate detector implementation approaches. For example, the ability to address individual pixels in a CMOS focal plane array enables dynamic switching to a selected region-of-interest, or a localized area of processing, with readout supported at high frame rates. In this case, the digital processing capability nominally used for an array of 1000×1000 pixels can be applied to a smaller field of view, for example to a 10×10 pixel area in order to support a significantly higher readout rate. For one example, a sub-array frame rate of 1000 frames per second (fps) is sought in order to close the sensing loop with a fast steering mirror as the actuator. The overall objective is to close the loop in a high bandwidth pointing and tracking test bed. The test bed will be used to test several performance metrics associated with a pointing acquisition and tracking scheme for orbital laser communications.
In particular, CMOS technology is allowing for more capable sensing and processing of data in a location removed from the host computer. The applications for CMOS detectors are increasing rapidly. However, only a few of the publications consider the application of these devices to spacecraft or space-borne systems. The reason appears to be the susceptibility of visible bandwidth CMOS focal plane array devices to total dose ionizing radiation damage, as well as proton displacement damage resulting in increased photodiode dark current. However, CMOS device scaling to sub-micron feature sizes have generally increased radiation hardness capability of both the CMOS circuitry as well as the photodiodes to much higher levels than those reported in the past. The goal of this test bed construction is to provide a means for independent analysis and application specific testing of visible silicon detector arrays with high-frame-rate readout and with region-of-interest capabilities.
The systems of the cited references disadvantageously require multiple sensors and processors, require sensor data pre-formatting and processing, have sensor limited field-of-view and poor output data, have slow millisecond pointing responses, have low pointing resolution in milliradians, do not process data in near-real time, require additional processing for laser jitter mitigation, are unable to reprogram internal sensor operations in near-real time, and require costly and complex optical technologies. These and other disadvantages are solved or reduced using the invention.